Inhibition of HIV entry is a promising strategy that has not yet been optimally exploited by current drugs. This proposal seeks to develop and characterize a novel class of protease-resistant D-peptides that bind to the highly conserved pocket region of HIV gp41 and block viral entry. D-peptides, composed of mirror-image D- amino acids, are not degraded by natural proteases. Therefore, they have the potential to persist in the body for extended periods of time compared to traditional L-peptide inhibitors, enabling dramatically lower and less frequent dosing at a much lower cost. PIE12-trimer is a highly potent D-peptide inhibitor that broadly inhibits all major HIV clades. It was designed with novel resistance capacitor, a reserve of binding energy that is predicted to provide a high genetic barrier to resistance. Indeed, the emergence of resistant strains is much slower for PIE12-trimer than earlier generation D-peptides or the approved entry inhibitor Fuzeon. This proposal will build on the promise of D-peptides and PIE12-trimer by designing and characterizing even more potent membrane-localized variants with pM potency. The mechanism by which HIV ultimately resists PIE12-trimer and earlier generation D-peptides will be characterized using deep sequencing to help predict clinical utility and inform the design of next-generation inhibitors. Novel high-throughput peptide screening techniques will be used to develop next-generation D-peptides that tolerate PIE12-trimer resistance mutations. These studies will advance D-peptide entry inhibitors for use as HIV preventative and therapeutic agents. Understanding how HIV resists this novel class of inhibitors will also improve our understanding of resistance mechanisms and guide the design of inhibitors with more robust resistance profiles. These studies will validate a rapid modular D-peptide design strategy that can be applied more broadly to inhibit protein- protein interactions for diverse biomedical applications, particularly emerging infectious diseases.